1. Field of the Invention
The subject matter of this invention concerns a process for the short-time treatment of plastic melt with a liquid treatment agent at a pressure and temperature at which the treatment agent is in the liquid state, with the mixture being subsequently expanded and subjected to degassing at a reduced pressure. The subject matter of this invention also concerns transparent thermoplastic materials which contain a poly(methacrylimide) plastic or transparent polymer mixtures produced therefrom with poly(methyl methacrylate).
2. Discussion of the Prior Art
A treatment process of the type mentioned above is employed, for example, to convert poly(methyl methacrylate) into poly(methacrylimide) plastics. According to the German Patent DE-C 2,652,118, poly(methyl methacrylate) is melted in an extruder, and ammonia or a primary amine is incorporated into the melt using pressure. These treatment agents, in combination with the polymerized units of methyl methacrylate, are converted into cyclic glutarimide units of the formula: ##STR1## in which R preferably stands for a hydrogen atom or a methyl group. In the course of this reaction, methanol is cleaved off and subsequently withdrawn from the extruder by evaporation under reduced pressure in the degassing zone of the extruder. The degree of this reaction, which is known as imidization, depends on the molar ratio between the ammonia or amine and the simultaneously present units of methyl methacrylate. With an excess of ammonia or amine, a considerable or complete imidization is obtained, while with substoichiometric quantities, a partial imidization is achieved. At the same time, units of methacrylic acid or methacrylic acid anhydride form.
It is known from the European Patent EP-A 216,505 that the partially imidized polymers are incompatible with one another as well as with poly(methyl methacrylate) and that they form turbid (opaque) mixtures. Even unmixed poly(methacrylimide) plastics are sometimes turbid, which indicates that they contain incompatible polymer constituents. According to the European Patent EP-A 376,748, the cause for this turbidity is to be seen in the distribution range of the degree of imidization which is more than 5%. Transparent poly(methacrylimide) plastics with a distribution range below 5% are obtained if the imidization is carried out in a solution of poly(methyl methacrylate) in an organic solvent, such as toluene. It is possible to achieve degrees of imidization of, for example, 75% to 80%. The drawback of this process is that the solvent must be evaporated after the reaction and must be separated from the cleaved methanol and the unreacted amine.
There are several processes in which melts of thermoplastic materials are mixed under pressure with highly liquid treatment agent. According to the German Patent DE-A 3,624,909, a liquid blowing agent is incorporated into a plastic melt, the mixture is cooled and expanded in the thermoelastic state to form a foamed plastic. In order to mix the components, the melt viscosities of which differ widely, a special mixing device is used, which is connected to an extruder. The mixing device is equipped with a rotor which is supported in a stator in such a way that a gap is formed, through which the mixture consisting of a blowing agent and thermoplastic materials can pass while the rotor is turning. The outside surface of the rotor and the inside surface of the stator are fitted with cavities that are arranged so as to overlap one another during the rotation of the rotor, thus causing the mixture consisting of the blowing agent and the thermoplastic materials to be transferred from one cavity to the next while the mixture passes through the mixing device.